If Thanos is the villain of 2, it's not the film Whedon wants to make. That's for sure. I'll take the word of the filmmaker as well as Marvel's desire to make this a long lasting universe over an easter egg, which could easily be a set up for a very long term plan.

Besides, you'll be waiting till the end of the world for Pym to show up. He's not going to.

And putting Thanos in the next one...there'd be no where to go but down from that. I'll stick to my prediction